supra molecular chemistry (English) - a field of chemistry that studies supramolecular structures (ensembles consisting of two or more molecules held together by means); "chemistry of molecular ensembles and intermolecular bonds" (definition by J.-M. Lena).

Description

Traditional chemistry is based on covalent bonds between atoms. At the same time, for the synthesis of complex nanosystems and molecular devices used in covalent chemistry not enough, because such systems can contain several thousand atoms. Intermolecular interactions come to the rescue - they help to combine individual molecules into complex ensembles called supramolecular structures.

The simplest example supramolecular structures are host–guest complexes. The host (receptor) is usually a large organic molecule with a cavity in the center, and the guest is a simpler molecule or ion. For example, cyclic polyesters of various sizes (crown ethers) bind ions rather strongly. alkali metals(Fig. 1).

Supramolecular structures are characterized by the following properties.

1. The presence of not one, but several binding centers in the host. In crown ethers, this role is played by oxygen atoms with unshared electron pairs.

2. Complementarity: geometric structures and electronic properties host and guest complement each other. In crown ethers, this manifests itself in the fact that the cavity diameter should correspond to the ion radius. Complementarity allows the host to perform selective binding of guests of a strictly defined structure. In supramolecular chemistry, this phenomenon is called "" (English - molecular recognition) (Fig. 2).

3. Complexes with a large number connections between complementary host and guest have a high structural organization.

Supramolecular structures are very widespread in nature. All reactions in living organisms proceed with the participation of protein catalysts. Enzymes are ideal host molecules. The active center of each enzyme is designed in such a way that only the substance (substrate) that corresponds to it in size and energy can get into it; the enzyme will not react with other substrates. Another example of supramolecular biochemical structures are molecules in which two polynucleotide chains are complementarily linked to each other through many hydrogen bonds. Each chain is both a guest and a host for the other chain.

The main types of non-covalent interactions that form supramolecular structures are ionic, and . All non-covalent interactions are weaker than covalent ones - their energy rarely reaches 100 kJ / mol, however big number bonds between the host and the guest ensures the high stability of supramolecular assemblies. Non-covalent interactions are weak individually but strong collectively.

The formation of supramolecular ensembles can occur spontaneously - this phenomenon is called. This is a process in which small molecular components spontaneously join together to form much larger and more complex supramolecular ones. During self-assembly, the entropy of the system decreases, Δ S

Δ G = Δ H – TΔ S

it is necessary that Δ H h| > | TΔ S|. This means that self-assembly occurs with the release of a large amount of heat. Home driving force self-assembly is the desire of chemical systems to lower the Gibbs energy through the formation of new chemical bonds, the enthalpy effect here prevails over the entropy one.

The main classes of supramolecular compounds are cavitands, cryptands, calixarenes, guest-host complexes, catenanes, . Supramolecular structures can also be attributed,.

Supramolecular chemistry methods find wide application in chemical analysis, medicine,

The development of the field of science called supramolecular chemistry is analyzed. The main definitions and concepts of this discipline are given. In a historical context, the studies that laid the foundations of supramolecular chemistry are considered. Examples of some of its typical objects, clathrates and cyclodextrins, are given. It is noted that the latest achievements in supramolecular chemistry and the most promising areas its uses are associated with the processes of self-assembly and self-organization, which, in particular, can be implemented in supramolecular synthesis and the creation of molecular and supramolecular devices.

Supramolecular chemistry. background

Supramolecular chemistry is one of the youngest and at the same time rapidly developing areas chemistry. For 25-30 years of its existence, it has already managed to pass a number of milestones, but at the same time, the basic ideas and concepts of this discipline are not yet well-known and generally accepted. In this review, we have sought to trace the development of the field of science called supramolecular chemistry, to identify the most successful definitions of its main tasks and the most important concepts and outline the current state and prospects.

The term "supramolecular chemistry" and the basic concepts of this discipline were introduced by the French scientist J.-M. Lenom in 1978 as part of the development and generalization of more early works(in particular, in 1973 the term “supermolecule” appeared in his works). Supramolecular chemistry was defined by the words: "Just as there is a field of molecular chemistry based on covalent bonds, there is also a field of supramolecular chemistry, the chemistry of molecular ensembles and intermolecular bonds." Subsequently, this first definition was reformulated many times. An example of another definition given by Len: “supramolecular chemistry is “a chemistry beyond the molecule” that studies the structure and function of associations of two or more chemical species held together by intermolecular forces” .

In many cases, the components that form supramolecular systems can be called (by analogy with the systems considered in molecular biology) molecular receptor and substrate, the latter being the smaller component that needs to be bound.

In order to adequately describe a chemical object, it is necessary to indicate its elements and the types of bonds between them, as well as spatial (geometric, topological) characteristics. The objects of supramolecular chemistry, the supermolecules, have the same certainty as the individual molecules that compose them. It can be said that "supermolecules are in relation to molecules what molecules are in relation to atoms, and the role of covalent bonds in supermolecules is played by intermolecular interactions".

According to Lehn, supramolecular chemistry can be broken down into two broad, overlapping areas:

– chemistry of supermolecules – clearly defined oligomolecular particles resulting from the intermolecular association of several components – the receptor and its substrate (substrates) and built on the principle of molecular recognition;

- chemistry of molecular ensembles - polymolecular systems that are formed as a result of spontaneous association of an indefinite number of components with a transition to a specific phase that has a more or less clearly defined microscopic organization and characteristics dependent on its nature (for example, clathrates, membranes, vesicles, micelles).

Supramolecular formations can be characterized by the spatial arrangement of the components, their architecture, "suprastructure", as well as the types of intermolecular interactions that hold the components together. Supramolecular ensembles have well-defined structural, conformational, thermodynamic, kinetic and dynamic properties; various types of interactions can be distinguished in them, differing in their strength, directionality, dependence on distances and angles: coordination interactions with metal ions, electrostatic forces, hydrogen bonds, van -der Waals interactions, donor-acceptor interactions, etc. The strength of interactions can vary in wide range, from weak or moderate, as in the formation of hydrogen bonds, to strong and very strong, as in the formation of coordination bonds with a metal. However, in general, intermolecular interactions are weaker than covalent bonds, so that supramolecular associates are less thermodynamically stable, kinetically more labile, and dynamically more flexible than molecules.

Thus, supramolecular chemistry embraces and makes it possible to consider from a unified standpoint all types of molecular associates, from the smallest possible (dimer) to the largest (organized phases). At the same time, it is necessary to emphasize once again that the objects of supramolecular chemistry necessarily contain parts (subsystems) that are not covalently bonded.

Lehn proposed to illustrate the transition from molecular to supramolecular chemistry with the scheme shown in Fig. one .

The main functions of supermolecules: molecular recognition, transformation (catalysis) and transfer. Functional supermolecules along with organized polymolecular assemblies and phases can be used to create molecular and supramolecular devices.

In addition to Lehn, one should also mention C. J. Pedersen and D. J. Crum, whose work and research played an important role in the development of supramolecular chemistry. In 1987, these three scientists were awarded the Nobel Prize in Chemistry (for their decisive contribution to the development of the chemistry of macroheterocyclic compounds capable of selectively forming host-guest molecular complexes).

Research that laid the foundations of supramolecular chemistry

The origins of the basic concepts of supramolecular chemistry can be found in works carried out in the past and at the very beginning of this century. So, P. Ehrlich in 1906 actually introduced the concepts of receptor and substrate, emphasizing that molecules do not react with each other if they do not first enter into a certain bond. However, the binding should not be any, but selective. This was emphasized by E. Fischer back in 1894, when he formulated his “key-lock” principle, a principle that suggests that steric correspondence, the geometric complementarity of the receptor and substrate, is the basis of molecular recognition. Finally, selective binding requires interaction, affinity between partners, and the roots of this idea can be sought in the works of A. Werner, which makes supramolecular chemistry in this respect a generalization and development of coordination chemistry.

According to J.-M. Len, these three concepts - fixation (binding), recognition and coordination - laid the foundation for supramolecular chemistry.

Some other concepts of supramolecular chemistry have also been known for a long time. Even the term « Übermolecule”, i.e. super-, or supermolecule, was introduced already in the mid-30s. of our century to describe a higher level of organization arising from the association of coordinatively saturated molecules (for example, during the formation of a dimer acetic acid). Was well known essential role supramolecular organization in biology.

However, the emergence and development of supramolecular chemistry as an independent field in the system chemical sciences happened much later. Here is what J.-M. Len in his book: “... for the emergence and rapid development new scientific discipline a combination of three conditions is required. First, it is necessary to recognize a new paradigm that shows the importance of disparate and at first glance not related observations, data, results and combining them into a single coherent whole. Secondly, tools are needed to study objects in this area, and here the development of modern technologies has played a decisive role for supramolecular chemistry. physical methods structure and properties studies (IR, UV and especially NMR spectroscopy, mass spectrometry, x-ray diffraction etc.), which make it possible to study even relatively labile supramolecular ensembles characterized by low-energy non-covalent interactions. Third, you need to be willing scientific community embrace the new paradigm so that new discipline could find a response not only among specialists directly involved in it, but also in close (and not very close) areas of science. This has happened with supramolecular chemistry, as far as one can judge from the rapid pace of its development and penetration into other disciplines over the past 25 years.

According to Lehn, "... supramolecular chemistry as we know it today began with the study of the selective binding of alkali metal cations by natural and synthetic macrocyclic and macropolycyclic ligands, crown ethers and cryptands" .

Among this kind of natural compounds, first of all, the antibiotic valinomycin should be pointed out. Deciphering its structure in 1963, in which huge contribution introduced by Soviet scientists led by Yu. A. Ovchinnikov, went far beyond the usual discovery. This cyclic depsipeptide (it is built from amino and hydroxy acid residues interconnected by amide and ester bonds) turned out to be the first among membrane-active complexons, or ionophores. Such names reflect the ability of these substances to form complex compounds with alkali cations in solutions and to transfer the bound cation through biological membranes. With the discovery of ionophores, the possibility of purposeful regulation of ion fluxes in living systems became a real possibility. For work in the field of membrane-active complexons, Ovchinnikov and his co-workers were awarded in 1978 Lenin Prize.

CHEMISTRY ORGANIC. MOLECULAR STRUCTURE
A. CHEMICAL BONDS OF CARBON
The chemical nature of carbon, intermediate between metals and typical non-metals, allows it to form covalent bonds with a large number of elements, most often with hydrogen, oxygen, nitrogen, halogens, sulfur and phosphorus. Carbon forms bonds with a high degree ionic nature with more electropositive metals, but such substances are highly reactive and are used as intermediates in the synthesis. Carbon-carbon bonds are covalent in nature and are simple (single), double, triple and aromatic
(see MOLECULE STRUCTURE).
aromatic systems. Benzene - the ancestor of the class of aromatic compounds - has a unique stability and enters into chemical reactions that are different from the reactions of non-aromatic systems. There are other aromatic systems, the most common of which have p-orbitals available for p-bond formation on each atom of the ring. Five-membered ring systems with two conjugated (ie, alternating with single) double bonds and a fifth atom carrying a lone pair of electrons are also aromatic in their properties. Below are some of these systems:

The concept of aromaticity was generalized by the German chemist E. Hückel. According to Hückel's rule, planar cyclic conjugated systems with 4n + 2 p-electrons are aromatic and stable, while the same systems with 4n p-electrons are antiaromatic and unstable.
Stability of cyclic systems. Valence angle (angle between bonds) in an unstressed fragment C-C-C is 109°, and rings that retain this value are more stable than those where the angles deviate greatly from this value. The stress that arises in cyclic systems as a result of the distortion of bond angles is called the Bayer voltage - after German chemist A. Bayer, who first proposed such an explanation for the stability of saturated rings. Thus, in three-membered rings, where the bond angle is only 60°, the rings are strongly strained and easily broken; some of their reactions resemble C=C double bond reactions. The four-membered rings are also strained (bond angle 90°), but not as strongly. The five-membered rings are almost flat and their angles are 108°; therefore they are unstressed and stable. In six-membered rings such as cyclohexane, the carbon atoms do not lie in the same plane; such cycles are folded, which reduces ring stress. Five- and six-membered rings are the most common. Large rings are also able to reduce angular stress by wrinkling, but in some of them (seven to twelve-membered) hydrogen atoms per opposite sides the rings approach so much that their repulsion makes the connection less stable (prelog voltage, named after the Swiss chemist V. Prelog, who discovered this effect).
Tautomerism. If a molecule or ion can be represented as several structures that differ from each other only in the distribution of electrons, these structures are called resonant, and the resonant forms are not in equilibrium with one another, just the actual electronic structure of the molecule is something in between these extremes. However, there are situations in which atoms move in a molecule when normal conditions so rapidly that an equilibrium is spontaneously established between the various molecular forms. This phenomenon is called tautomerism. An example is the equilibrium between ketone and enol (keto-enol tautomerism):

Here, the two compounds differ only in the arrangement of the hydrogen cation and the pair of electrons (in the p-bond). Equilibrium is established quickly, but is strongly shifted towards the keto form. Therefore, alcohols with the -C=C-OH structure are usually unstable and quickly turn into the keto form, unless there are some structural features that stabilize the enol form, for example, in phenols, which would lose their aromatic character upon transition to the keto form:

Tautomerism is common in molecules that have the structure -CH=X or -C=XH, where X is S, O, or N. Thus, the H2C=C(NH2)-CH3 molecule rapidly rearranges to H3C-C(=NH)- CH3, and R-C(OH)=NH imides rearrange into R-C(=O)NH2 amides. Tautomerism is common in such biologically important heterocyclic systems as barbituric acid and related compounds:

Such substances in tautomeric equilibrium often enter into reactions characteristic of both forms.
Other fast equilibria. Other fast equilibria between molecules with related structures are also known. If any two of the OH, SH, or NH2 groups are on the same carbon atom, the compound is usually unstable compared to the doubly bonded form:

There are cases where this equilibrium is shifted towards the dihydroxy compound. Gaseous formaldehyde has the structure CH2=O, but in aqueous solution it attaches a water molecule, acquiring HO-CH2-OH as the predominant form. Chloral hydrate Cl3CCH(OH)2 is stable in the dihydroxyl form as a result of the electron-withdrawing effect of three chlorine atoms.
B. ISOMERIA
Isomerism of the carbon chain. Molecules that differ only in the branching of the carbon chain are called chain isomers. An example has already been given - this is an isomeric pair of n-butane and isobutane.
isomerism functional groups. Molecules with the same gross formulas but different functional groups are functional isomers, for example ethanol C2H5OH and dimethyl ether CH3-O-CH3.
Position isomerism. Positional isomers have the same gross formulas and functional groups, but the positions of functional groups in their molecules are different. Thus, 1-chloropropane CH3CH2CH2Cl and 2-chloropropane CH3CHClCH3 are positional isomers.
Geometric isomerism. Geometric isomers consist of identical atoms connected in the same sequence, but differ in the spatial arrangement of these atoms relative to double bonds or rings. The cis-trans isomerism of olefins and the syn-anti-isomerism of oximes are of this type.

Optical isomerism. Molecules are called optical isomers when they are made up of identical atoms connected in the same way, but differ in the spatial arrangement of these atoms in the same way as right hand different from the left. Such isomerism is possible only when the molecule is asymmetric, i.e. when it does not have a plane of symmetry. The simplest way to such a situation - the attachment of four different groups to the carbon atom. Then the molecule becomes asymmetric and exists in two isomeric forms. The molecules differ only in the order of attachment to the central carbon atom, which is called an asymmetric carbon atom or a chiral center, since it is connected to four different groups. Note that the two optical isomers are mirror reflection each other; they are called "enantiomers" or "optical antipodes" and have the same physical and Chemical properties, except that they rotate the plane of polarized light in opposite directions and react differently with compounds that are themselves optical isomers. An isomer that rotates the plane of polarized light clockwise is called the d- (from "dextro" - right) or (+)-isomer; The isomer that rotates light counterclockwise is called the l- (from "left" - left) or (-)-isomer. When more than one asymmetric center is present in a molecule, the maximum possible number of optical isomers is 2n, where n is the number of asymmetric centers. Sometimes some of these isomers are identical, and this reduces the number of optical isomers. Thus, meso-isomers are optical isomers that are optically inactive because they have a plane of symmetry. Optical isomers that are not mirror images are called "diastereomers"; they differ in physical and chemical properties in the same way that geometric isomers differ in them. These differences can be illustrated by the example of six-carbon straight-chain sugars having following structure: CH2OH-*CHOH-*CHOH-*CHOH-*CHOH-CHO. Here, four asymmetric atoms, marked with an asterisk, are each connected to four different groups; thus, 24, or 16, isomers are possible. These 16 isomers make up 8 pairs of enantiomers; any pair that is not enantiomers are diastereomers. Six of these 16 sugars are presented below as so-called. Fisher projections.

The designations D- and L- for enantiomers do not refer to the direction of rotation (denoted d or l), but to the position of the OH at the lowest (in the Fischer projection) asymmetric carbon: when OH is on the right, the isomer is denoted as D, when on the left, L. D - and L-forms of glucose have the same melting points, solubility, etc. On the other hand, glucose and galactose, being diastereomers, have various points melting, solubility, etc.

Collier Encyclopedia. - Open society. 2000 .

See what "ORGANIC CHEMISTRY. MOLECULAR STRUCTURE" is in other dictionaries:

Collier Encyclopedia

The branch of chemistry that studies carbon compounds, which include, firstly, the substances that make up most living matter (proteins, fats, carbohydrates, nucleic acids, vitamins, terpenes, alkaloids, etc.); secondly, many substances, ... ... Collier Encyclopedia

This term has other meanings, see Chemistry (meanings). Chemistry (from Arabic کيمياء‎‎, which presumably originated from the Egyptian word km.t (black), from where the name of Egypt, black soil and lead “black ... ... Wikipedia

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The chemistry of molecules such as C2H2, N2H2 and H202 is determined by the orbitals formed by the combination of px - and py - A. In linear acetylene, these orbitals give rise to filled tsv - and free icg - orbitals (ch. The molecule, of course, has axial symmetry.

The chemistry of the carbon monoxide molecule can in part be well explained by this form, the equivalent of which in terms of molecular orbital theory is not considered here. In this structure, carbon has an isolated pair of electrons and one empty orbit, since the carbon nucleus is surrounded by only a sextet of electrons instead of the usual octet. Based on these considerations, it can be expected that carbon monoxide is also capable of interacting with nucleophilic groups, such as bases, which can be a source of electrons to fill an octet. Indeed, similar carbon monoxide reactions are known; some of them will also be discussed below.

The chemistry of molecules continues to be modern organic chemistry. However, for inorganic compounds, the molecular form of the existence of a substance is characteristic only for the gas and vapor state.

The chemistry of molecules continues to be modern organic chemistry, and most inorganic substances do not have molecular structure. AT last case macrobodies are composed either of atoms of the same chemical element, or from atoms different elements. Recognition of the non-molecular form of the existence of a solid substance leads to the need to revise some of the provisions of chemical atomistics, to modernize the basic laws and concepts that are valid for pneumatic (gas) chemistry.

In molecular chemistry, there are two fundamentals.

As in molecular chemistry, exothermic and endothermic reactions. The determination of the magnitude and sign of the thermal effect of reactions can be carried out using the law of equivalence of mass and energy.

Group theory is applied here much more widely than in molecular chemistry. At the same time, the ability to derive the regularities of the particle mass spectrum from fundamental principles, say, from geometrodynamics, is much more problematic here than the ability to calculate the binding energy of a molecule using the Schrödinger equation.

Such a desire to spread the ideas and theories that have grown in the bowels of organic chemistry(chemistry of molecules), to the field of inorganic chemistry, as it is now clear to us, turned out to be illegal mainly because inorganic compounds are, as a rule, non-molecular systems. In the same systems, not covalent, but ionic bonds. The distinctive feature complex compounds is that they are compounds of molecules, not atoms.

At first, it was only about cofactors, which, however, were often found on the basis of the analysis of crystalline structural associations and transferred to molecular chemistry, despite the lack of material on the relationship between molecular chemistry and crystal chemistry. For compounds of certain classes, these relations are so simple that they allow certain valences to be preassigned to the particles, from which the actual coefficients can be derived. It must not be overlooked that this regularity (which, owing to numerous difficulties, cannot be taken for granted for chemical compounds in general) so quickly gained recognition only for a geochemical reason. Oxygen is essential element outer lithosphere, and precisely on the basis of the relationship between the numbers of oxygen atoms and other elements in oxygen compounds a rule was deduced that any stoichiometric ratios are impossible for electrically neutral associations.

At first, it was only about coefficients, which, however, were often found on the basis of an analysis of crystalline structural associations and transferred to molecular chemistry, despite the lack of material on the relationship between molecular chemistry and crystal chemistry. At the present time, we can assume that in electrically neutral atomic associations, certain types of atoms under normal conditions stand in simple stoichiometric ratios to others. For compounds of certain classes, these relations are so simple that they allow certain valences to be preassigned to the particles, from which the actual coefficients can be derived. It must not be overlooked that this regularity (which, owing to numerous difficulties, cannot be taken for granted for chemical compounds in general) was recognized so quickly only for a geochemical reason. Oxygen is the most important element of the outer lithosphere, and it was precisely on the basis of the ratios between the numbers of oxygen atoms and other elements in oxygen compounds that the rule was derived that any stoichiometric ratios are impossible for electrically neutral associations.

Manifestations of electronic-vibrational (or, in short, vibronic) interactions in polyatomic systems, which are combined in the literature under common name the Jahn-Teller effect, form at present a new rapidly developing direction in the physics and chemistry of molecules and crystals.

It is easy to see that between reactions I and II there are fundamental differences. Reaction I represent the chemistry of molecules; only energy factors participate in the activation of their reagents. Reactions II represent the whole chemistry, the change of molecules in which is carried out mainly with the participation of berthollide systems. The direction and rate of reactions I are determined entirely by the chemical structure of the reacting molecules. The direction and rate of reactions II are determined both by the chemical structure of the reacting molecules and chemical organization catalytic system.

Since for the formation of a ring, closed hydrogen bonds, necessary excited state, apparently, there is no point in studying in detail the effect of various groups of substituents on the cyclization process using data on conventional reactions of organic chemistry. It can be said that photochemistry is concerned with the chemistry of molecules in an excited state rather than in the ground state.

I wanted to dwell briefly on the question of the reactivity of molecules in the triplet state. This question, generally speaking, is very large, since the chemistry of molecules in the triplet state is an independent field. I will dwell only on the qualitative characteristic of the activation energy of a reaction involving molecules in the triplet state. What is the difference between such a molecule and a radical. The simplest example is the O2 molecule, for which the triplet state is the ground state. AT this case Obviously, there is no activation energy.

Chemistry, the basic concepts of which we will consider, is a science that studies substances and their transformations that occur with a change in structure and composition, and hence properties. First of all, it is necessary to define what such a term as "substance" means. If we talk about it in broad sense, it is a form of matter that has a rest mass. Any substance is elementary particle, for example, a neutron. In chemistry, this concept is used in a narrower sense.

To begin with, let us briefly describe the basic terms and concepts of chemistry, atomic and molecular science. After that, we will explain them, and also outline some important laws of this science.

The basic concepts of chemistry (substance, atom, molecule) are familiar to each of us from school. Below is given a brief description of them, as well as other, not so obvious terms and phenomena.

atoms

First of all, all substances that are studied in chemistry are made up of small particles called atoms. Neutrons are not the object of study of this science. It should also be said that atoms can combine with each other, resulting in the formation of chemical bonds. In order to break this bond, an expenditure of energy is required. Consequently, atoms do not exist individually under normal conditions (with the exception of "noble gases"). They connect with each other at least in pairs.

Continuous thermal motion

Continuous thermal motion characterizes all particles that are studied by chemistry. The basic concepts of this science cannot be stated without talking about it. With continuous motion of particles, it is proportional to temperature (however, it should be noted that the energies of individual particles are different). Ekin = kT / 2, where k - Boltzmann's constant. This formula is valid for any kind of movement. Since Ekin = mV 2 / 2, the motion of massive particles is slower. For example, if the temperature is the same, oxygen molecules move on average 4 times slower than carbon molecules. This is because their mass is 16 times greater. Movement is oscillatory, translational and rotational. Vibrational is observed in both liquid and solid, and in gaseous substances. But translational and rotational is most easily carried out in gases. In liquids it is more difficult, but in solids- even more difficult.

molecules

We continue to describe the basic concepts and definitions of chemistry. If atoms combine with each other, forming small groups (they are called molecules), such groups take part in thermal motion, acting as a single whole. Up to 100 atoms are present in typical molecules, and their number in the so-called macromolecular compounds can reach 105.

Non-molecular substances

However, atoms are often united in huge collectives from 107 to 1027. In this form, they practically do not take part in thermal motion. These associations bear little resemblance to molecules. They are more like pieces. solid body. These substances are usually called non-molecular. In this case thermal motion is carried out inside the piece, but it does not fly, like a molecule. There are also transition region sizes, which includes associations consisting of atoms in an amount from 105 to 107. These particles are or very large molecules, or are small grains of powder.

ions

It should be noted that atoms and their groups can have electric charge. In this case, they are called ions in a science such as chemistry, the basic concepts of which we study. Since charges of the same name always repel each other, a substance where there is a significant excess of certain charges cannot be stable. Negative and positive charges always alternate in space. And the substance as a whole remains electrically neutral. Note that the charges, which are considered large in electrostatics, are negligible from the point of view of chemistry (for 105-1015 atoms - 1e).

Objects of study in chemistry

It should be clarified that the objects of study in chemistry are those phenomena in which atoms do not arise and are not destroyed, but only regroup, that is, they combine in a new way. Some links are broken, resulting in the formation of others. In other words, new substances appear from the atoms that were in the composition starting materials. If, however, both atoms and the bonds existing between them are preserved (for example, during the evaporation of molecular substances), then these processes are no longer the field of study of chemistry, but of molecular physics. In the case when atoms are formed or destroyed, we are talking about the subjects of study of nuclear or atomic physics. However, the boundary between chemical and physical phenomena blurred. After all, the division into individual sciences conditional, while nature is indivisible. Therefore, the knowledge of physics is very useful for chemists.

We briefly outlined the basic concepts of chemistry. Now we invite you to consider them in more detail.

More about atoms

Atoms and molecules are what many associate chemistry with. These basic concepts must be clearly defined. The fact that atoms exist was brilliantly guessed two thousand years ago. Then, already in the 19th century, scientists had experimental data (still indirect). It's about about Avogadro's multiple ratios, the laws of constancy of composition (below we will consider these basic concepts of chemistry). The atom continued to be explored in the 20th century, when many direct experimental evidence. They were based on spectroscopy data, on the scattering of X-rays, alpha particles, neutrons, electrons, etc. The size of these particles is approximately 1 E = 1o -10 m. Their mass is about 10 -27 - 10 -25 kg. At the center of these particles is a positively charged nucleus, around which electrons with a negative charge move. The core size is about 10 -15 m. It turns out that electron shell determines the size of an atom, but its mass is almost completely concentrated in the nucleus. One more definition should be introduced, considering the basic concepts of chemistry. A type of atom that has the same nuclear charge.

It is often found as the smallest particle of a substance, chemically indivisible. As we have already noted, the division of phenomena into physical and chemical is conditional. But the existence of atoms is unconditional. Therefore, it is better to define chemistry through them, and not vice versa, atoms through chemistry.

chemical bond

This is what keeps the atoms together. It does not allow them to scatter under the influence of thermal motion. We note the main characteristics of bonds - this is the internuclear distance and energy. These are also the basic concepts of chemistry. The bond length is determined experimentally with a sufficiently high accuracy. Energy - too, but not always. For example, it is impossible to objectively determine what it is in relation to separate connection in a complex molecule. However, the atomization energy of a substance, necessary to break all existing bonds, is always determined. Knowing the bond length, you can determine which atoms are bonded (they have a short distance) and which are not (they have a long distance).

Coordination number and coordination

Basic concepts analytical chemistry includes these two terms. What do they stand for? Let's figure it out.

The coordination number is the number of nearest neighbors of a given specific atom. In other words, this is the number of those with whom he is chemically associated. Coordination is mutual arrangement, type and number of neighbors. In other words, this concept is more meaningful. For example, the coordination number of nitrogen, characteristic of the molecules of ammonia and nitric acid, the same - 3. However, their coordination is different - non-planar and flat. It is determined regardless of ideas about the nature of the bond, while the degree of oxidation and valency are conditional concepts that are created in order to predict coordination and composition in advance.

Molecule definition

We have already touched on this concept, considering the basic concepts and laws of chemistry briefly. Now let's dwell on it in more detail. Textbooks often define a molecule as the smallest neutral particle of a substance that has its chemical properties and is also able to exist independently. It should be noted that this definition is this moment already outdated. First, what all physicists and chemists call a molecule does not preserve the properties of matter. Water dissociates, but this requires a minimum of 2 molecules. The degree of dissociation of water is 10 -7 . In other words, only one molecule out of 10 million can undergo this process. If you have one molecule, or even a hundred, you will not be able to get an idea of ​​​​its dissociation. The fact is that thermal effects Reactions in chemistry usually involve the energy of interaction between molecules. Therefore, they cannot be found by one of them. Both chemical and physical substances can only be determined from a large group of molecules. In addition, there are substances in which the "smallest" particle capable of existing independently is indefinitely large and very different from the usual molecules. A molecule is actually a group of atoms that is not electrically charged. In a particular case, this may be one atom, for example, Ne. This group must be able to participate in diffusion, as well as in other types of thermal motion, acting as a whole.

As you can see, the basic concepts of chemistry are not so simple. A molecule is something that needs to be carefully studied. It has its own properties as well as molecular weight. We will talk about the latter now.

Molecular mass

How to determine the molecular weight experimentally? One way is based on Avogadro's law, according to relative density pair. The most accurate method is mass spectrometric. An electron is knocked out of a molecule. The resulting ion is first accelerated in an electric field, then deflected magnetically. The ratio of charge to mass is determined precisely by the magnitude of the deviation. There are also methods based on the properties that solutions have. However, molecules in all these cases must certainly be in motion - in solution, in vacuum, in gas. If they are not moving, it is impossible to objectively calculate their mass. And their very existence in this case is difficult to detect.

Features of non-molecular substances

Speaking of them, they note that they are composed of atoms, not molecules. However, the same is true for noble gases. These atoms move freely, therefore, it is better to think of them as monatomic molecules. However, this is not the main thing. More importantly, in non-molecular substances there are a lot of atoms that are connected together. It should be noted that the division of all substances into non-molecular and molecular is insufficient. The division by connectivity is more meaningful. Consider, for example, the difference in the properties of graphite and diamond. Both are carbon, but the former is soft and the latter is hard. How do they differ from each other? The difference lies precisely in their connectivity. If we consider the structure of graphite, we will see that strong bonds exist only in two dimensions. But in the third, interatomic distances are very significant, therefore, there is no strong bond. Graphite easily slides and splits over these layers.

Structure Connectivity

Otherwise, it is called spatial dimension. It represents the number of dimensions of space, characterized by the fact that they have a continuous (almost infinite) core system ( strong ties). The values ​​it can take are 0, 1, 2 and 3. Therefore, it is necessary to distinguish between three-dimensionally connected, layered, chain and island (molecular) structures.

Law of constancy of composition

We have already learned the basic concepts of chemistry. The substance was briefly reviewed by us. Now let's talk about the law that applies to him. It is usually formulated as follows: any individual substance (that is, pure), regardless of how it was obtained, has the same quantitative and qualitative composition. But what does the concept mean? Let's see.

Two thousand years ago, when the structure of substances could not yet be studied by direct methods, when the basic chemical concepts and laws of chemistry familiar to us did not even exist yet, it was determined descriptively. For example, water is the liquid that forms the basis of the seas and rivers. It has no smell, color, taste. It has such and such freezing and melting temperatures, it turns blue. Salty sea water is because it is not clean. However, the salts can be separated by distillation. More or less like this, descriptive method, the basic chemical concepts and laws of chemistry were determined.

For scientists time, it was not obvious that the liquid that was released different ways(hydrogen combustion, vitriol dehydration, sea water distillation), has the same composition. A great discovery in science was the proof of this fact. It became clear that the ratio of oxygen and hydrogen cannot change smoothly. This means that the elements are made up of atoms - indivisible portions. So the formulas of substances were obtained, and the idea of ​​scientists about molecules was justified.

Nowadays, any substance is explicitly or implicitly defined primarily by the formula, and not by melting point, taste or color. Water is H 2 O. If other molecules are present, it will no longer be pure. Therefore, pure molecular substance is one that is composed of molecules of only one type.

However, what about electrolytes in this case? After all, they contain ions, not just molecules. A more rigorous definition is needed. A pure molecular substance is one that is composed of molecules of the same type, and also, possibly, the products of their reversible rapid transformation (isomerization, association, dissociation). The word "fast" in this context means that we cannot get rid of these products, they immediately appear again. The word "reversible" indicates that the transformation is not completed. If brought, then it is better to say that it is unstable. In this case, it is not a pure substance.

The law of conservation of mass of matter

This law has been known in metaphorical form since ancient times. He said that matter is uncreatable and indestructible. Then came its quantitative formulation. According to it, weight (and from the end of the 17th century - mass) is a measure of the amount of a substance.

This law in its usual form was discovered in 1748 by Lomonosov. In 1789, it was supplemented by A. Lavoisier, a French scientist. Its modern formulation sounds like this: the mass of substances entering into a chemical reaction is equal to the mass of substances that are obtained as a result of it.

Avogadro's law, the law of volumetric ratios of gases

The last of these was formulated in 1808 by J. L. Gay-Lussac, a French scientist. This law is now known as Gay-Lussac's law. According to him, the volumes of reacting gases are related to each other, as well as to the volumes of the resulting gaseous products, as small integers.

The pattern that Gay-Lussac discovered explains the law that was discovered a little later, in 1811, by Amedeo Avogadro, an Italian scientist. It states that under equal conditions (pressure and temperature) in gases having the same volumes, there is the same number of molecules.

Two important consequences follow from Avogadro's law. The first is that when same conditions one mole of any gas occupies an equal volume. The volume of any of them under normal conditions (which are a temperature of 0 ° C, as well as a pressure of 101.325 kPa) is 22.4 liters. The second consequence of this law is the following: under equal conditions, the ratio of the masses of gases having the same volume is equal to the ratio of their molar masses.

There is another law, which must certainly be mentioned. Let's talk about it briefly.

Periodic law and table

D. I. Mendeleev, based on the chemical properties of elements and atomic and molecular science discovered this law. This event took place on March 1, 1869. Periodic Law is one of the most important in nature. It can be formulated as follows: the properties of the elements and the complex and simple substances have a periodic dependence on the charges of the nuclei of their atoms.

The periodic table created by Mendeleev consists of seven periods and eight groups. Groups are its vertical columns. The elements within each of them have similar physical and chemical properties. The group, in turn, is divided into subgroups (main and secondary).

The horizontal rows of this table are called periods. The elements that are in them differ from each other, but they also have in common - that their last electrons are located on the same energy level. There are only two elements in the first period. These are hydrogen H and helium He. There are eight elements in the second period. There are already 18 of them in the fourth. Mendeleev designated this period as the first big one. The fifth also has 18 elements, its structure is similar to the fourth. The sixth one contains 32 elements. The seventh is not completed. This period starts with francium (Fr). We can assume that it will contain 32 elements, like the sixth one. However, only 24 have been found so far.

Rollback rule

According to the rollback rule, all elements tend to gain or lose an electron in order to have the 8-electron noble gas configuration closest to them. Ionization energy is the amount of energy required to separate an electron from an atom. The rollback rule states that when moving from left to right on periodic table necessary more energy to remove an electron. Therefore, elements on the left side tend to lose an electron. On the contrary, those located with right side want to buy it.

We briefly outlined the laws and basic concepts of chemistry. Certainly, this is only general information. Within the framework of one article it is impossible to talk in detail about such a serious science. The basic concepts and laws of chemistry, summarized in our article, are only a starting point for further study. Indeed, in this science there are many sections. There is, for example, organic inorganic chemistry. The basic concepts of each of the sections of this science can be studied for a very long time. But those presented above refer to general issues. Therefore, we can say that these are the basic concepts of organic chemistry, as well as inorganic.